does it improve if you squeeze the primer bulb? if so its probaly a fuel problem where to start?
1/when was it last serviced? if recent was al the filters changed was the fuel pump diaphrams changed ?
2/you would need to put preassure gauges on the fuel system and see what preassures you have on both systems I have seen the low preasure fuel pump failure cause this
3/ is any spark plug a differant colour from the rest?
4/ rib shop is a good one fidally to do
5/ has the fuel line been dis-connected from the fuel tank if so it should have a vacumn test for air leacks , I used a piece of clear plastic hose about six inches long , making sure all fuel connections are tight.
6/ what was you going to do on sunday
7/ take to a dealer and get him to put it on his diagnostic equipment

150 optis have a recall on coils going down, pull the plug leads to find a missfire, swap coils and see if problem shifts cylinders, Can be done with dealer CDS (Iv just done one that would only rev to 3500rpm)
you have to change all coils not only faulty ones. Covered by unlimited time warrenty.
If you dond find a missfire when you pull the plug leads it could be the shift switch under the oil res bottle at the bottom of the cowl at the front. if it does not recognise its in gear it will limit the revs. again easy with CDS